Why your DC motor might fail

14 January 2018

Control Engineering UK finds out more about the factors that have a role to play in DC motor failure.

The effect of load torque on the thermal time constant of the winding.
The effect of load torque on the thermal time constant of the winding.

The most important element of ensuring long-service life for your DC motors is to handle them with care. If a motor is dropped it could dislodge small components within it or even fracture them.

Another important factor to consider is electro-static discharge (ESD), which is critical for brushless motors. ESD could damage the hall sensors or encoder, rendering them useless and unable to get any feedback from the control system. Brushless motors commutate electrically through block or sinusoidal commutation. Studies show that it is possible to generate up to 15,000 volts in the human body. By releasing that energy you could short-circuit a motor which is why wearing the correct gear and thinking about how you are going to handle the motor is crucial.

Environmental conditions need to be considered before specifying a motor. Each motor will have its own operating temperature range and this is dictated by the heat reliability of the materials and the viscosity of the lubricant in the bearings. If the ambient temperature falls outside this range, then the motor may fail sooner than you expect. At low temperature the lubricant will freeze and increase friction within the bearing causing increasing wear and excessive axial play due to shrinkage of components. You will have the same consequences operating at high temperatures. However, in this case, the lubricant will become a liquid and will run away from the areas where lubricant is needed. Another element that might need to be considered is the humidity of the environment in which it operates. High levels of humidity can cause corrosion within the bearing. This increases friction which has a growing effect on wear. This may damage the shaft and the bearings restricting the smooth rotating operation of the motor which can affect speed and torque. It also will lead to contamination within the drive due to the abrasive nature that is corrosion.

DC motors commutate using brushes, whether it be graphite or precious metal. It is the brushes that limit the life of these motors and it is the ball bearings that limits brushless motors. The life of the brushes can be influenced by a number of factors in addition to humidity and vibration. The use of high current loads will cause electrical wear; high speeds will considerably impact the mechanical wear on the brushes; the type of operation can significantly deteriorate the brushes if the motor is used for extreme start/stop and left/right operations and temperature. All these factors contribute to the demise of a brushed DC motor. Brushless motors will fail once the bearing has worn out, but in addition, residual unbalancing and bearing load have an impact on bearing life.

Data for each motor will specify its limits and operating ranges. This data is not always taken into account. A key parameter is the thermal time constant of the winding. DC motors can be overrun, but not for long periods of time. The thermal time constant of the winding (tw) is what dictates the maximum operating time for an overload. The maxon motor DCX 6 M, for example said to have a  tw of 1.71 seconds, so that motor should only operate for that period while overloaded by around 2.5 times the nominal torque. (In maxon motors, torque is directly proportional to the current, so when torque increases, it has a higher electrical current load demand). By operating for longer periods, the winding will melt due to the excessive current which is producing large amounts of heat leading to the motor seizing.

Factors that can lead to failure include:
• Human interaction
• Environment conditions
• Brushes
• Ball bearings
• Exceeding operating range

Information to create this article was provided by Maxon Motor.

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